1. Field of the Invention
The present invention relates to a device of a simple structure for improving the combustion of fresh mixture in four cycle internal combustion engine, through enhancing the combustion efficiency by generating a strong swirling flow of combustion gas which is sucked back into a combustion chamber from an exhaust passage during suction stroke, to improve the performance of the engine over whole range of engine operation from full load to light load range and to diminish as much as possible the generation of unburnt noxious components such as HC, CO and so forth.
2. Description of the Prior Art
In the operation of the four cycle internal combustion engine for automobiles, it is a common measure for obtaining a higher suction efficiency to make the opening periods of the intake and exhaust valves overlap each other. More specifically, the intake valve starts to open just before the ending of the exhaust stroke in which the piston ascends and the exhaust valve is kept opened until an intermediate point of the subsequent suction stroke in which the piston descends. This overlap of the valve opening periods can provide the desired large engine output through enhancing the suction efficiency without being accompanied by deterioration of combustion of the mixture in the heavy load region in which the absolute amount of the intake air is large.
However, in the light load region in which the opening degree of the throttle valve is small and the absolute amount of the intake air is correspondingly small, the ratio of the amount of residual gas to the amount of the fresh intake mixture in each stroke of the engine is considerably high. In fact, the volume of the fresh mixture is as small as 20 to 40% of the cylinder volume. In consequence, the ignition is made unstable and the generation of disturbance of the fresh mixture, caused by the reciprocation of the piston and squish, is suppressed and, further, the speed of propagation of flame is lowered to incur a reduction of the combustion efficiency. In consequence, the generation of unburnt noxious components in the exhaust gas, such as HC, CO and so forth, is undesirably promoted. This tendency is serious particularly in the large-output type engine having long period of overlap of the opening of intake and exhaust valves. Thus, the large output type engine inherently involves a problem that the combustion efficiency is lowered and the generation of noxious unburnt components such as HC, CO and so forth is promoted in the light load region of the engine operation.
In order to overcome this problem, there have been proposed various measures for improving the combustion by generating a swirling or disturbance of the mixture in the combustion chamber by, for example, making the intake passage in a spiral form, forming a vane on the intake valve, or forming a guide wall around the intake valve. These measures for generating a swirling or disturbance of the mixture, however, increase the resistance to the flow of the mixture during heavy load operation, resulting in a reduction of the suction efficiency to hinder the increase of the engine output. Thus, the desired large engine output cannot be obtained in the heavy load region of the engine operation.
Further, during the deceleration of the engine in which the throttle valve is in the idle position while the engine speed is still high, the amount of fresh mixture is extremely small and the ratio of the amount of residual gas to the fresh mixture in the cylinder is so large that the combustion cannot be improved sufficiently solely by the aforementioned measures for generating a swirling or disturbance of the mixture.
Hitherto there have been proposed various measures for improving the combustion in the deceleration period of the engine operation, such as provision of a throttle opener or a dash pot in the intake system in order to temporarily increasing the opening of the throttle valve. These measures are effective in improving the combustion during deceleration of the engine by increasing the amount of fresh mixture to increase the ratio of the latter to the residual gas in the cylinder, but pose, on the other hand, a problem that fresh mixture is unnecessarily introduced into the engine during deceleration to undesirably promote the combustion resulting in a deterioration of the engine braking performance and, hence, in the degradation of the driveability during deceleration.